Mobile communication devices are in widespread use throughout metropolitan regions of the world. These devices are increasingly common and affordable, and to remain competitive manufactures have sought to include additional functionality in them. For example, manufacturers are now including positioning receivers in mobile communication devices to support a number of location applications, such as location reporting for emergency services, and navigation.
The use of satellite positioning receivers in mobile communication devices was initially driven by safety concerns related to locating a person calling emergency services using a mobile communication device. Although it is simple to determine the cell location of a mobile caller, the area encompassed by a cell may be quite large, especially in rural areas. Therefore some governments are now requiring that mobile communication devices include a means for determining their own location and reporting it when necessary, such as when the mobile communication device user calls an emergency phone number. Although numerous methods of approximate location determination have been developed, using such techniques as triangulation and relative power levels of signals received from base station radios in the vicinity of the mobile communication device, these methods have not proven sufficiently reliable or precise.
The preferred means of providing location determination in a mobile communication device is to include a satellite positioning receiver. However, this approach is not without significant design challenges. For one, the time needed for a satellite positioning receiver to lock onto a sufficient number of positioning satellite signals from an autonomous or “cold” start can be significant, taking several minutes or more. This is due to the number of unknown variables the satellite positioning receiver must determine when commencing a cold start. For one, the satellite positioning receiver must know which satellites are presently in view to search for the signals transmitted by those satellites, otherwise the positioning receiver must search for each satellite until it determines which ones are presently in view. The ephemeris orbital parameters, and related parameters such as Doppler shift are of significant assistance if known before initiating a location fix. The present time of day and approximate location are very helpful. This type of aiding information may be available from a communication system, and may be transmitted to the mobile communication device from, for example, a base station in a cellular telephony system. Aiding information received from the network is referred to as network aiding. Network aiding will reduce the time to first fix to a few seconds to a minute, depending on signal conditions. Network aiding involves transmitting information to reduce the search time of the satellite positioning receiver, and may include time of day, approximate location, and satellite ephemeris. However, when the mobile communication device is out of range of the communication system, is not designed to access a communication system, or when the communication system does not support satellite positioning location aiding, the mobile communication device must often perform a cold start in determining its location.
However, some mobile communication devices are being manufactured with the ability to communicate directly with other mobile communication devices for use in remote areas, outside of communication system coverage. In situations where the mobile communication device is outside the coverage area of a communication system, the precision frequency signal is unavailable. Therefore there is a need for a way in which a mobile communication device operating in a remote area outside the coverage of a communication system can reduce the time needed to perform a location determination.